Plastics used for optical materials such as optical disk substrates and optical lenses require a number of properties, in addition to transparency, including optical isotropy (low birefringence), dimensional stability, weather resistance and thermal stability. Polycarbonates and polymethyl methacrylates have mainly been used for such optical uses in the past, but polycarbonates have disadvantages including a large intrinsic birefringence and a tendency toward optical anisotropy of the molded products, while polymethyl methacrylates also have disadvantages such as poor dimensional stability, due to their extremely high water absorption, and low heat resistance.
Presently, optical disk substrates employ polycarbonates almost exclusively and, with the recent progress in increased capacity magnetic optical disks (MODS) and high recording density, as typified by the development of digital video disks (DVDs), problems such as the degree of the birefringence of polycarbonates and warping of disks by moisture absorption have become matters of concern.
In light of these circumstances, development has been accelerating in the area of cyclic olefin polymers as substituting materials for polycarbonates. Production processes for these polymers can largely be classified into the following 2 types.
(1) The cyclic olefin is subjected to ring opening polymerization with a metathesis catalyst, after which the resulting unsaturated double bonds on the main polymer chain are hydrogenated. PA0 (2) A Ziegler-Natta catalyst or Kaminsky catalyst is used for copolymerization of an .alpha.-olefin such as ethylene with a cyclic olefin, without ring opening of the cyclic olefin.
The advantage of production process (1) is that, since the primary structure of the polymer is uniformly established, high chemical homogeneity is achieved to result in polymers with high transparency when molded into articles; however, costly polycyclic olefins must be used as the monomers in order to achieve high heat resistance. For example, such olefins as are commercially available at the present time include the amorphous polyolefin resin [tradename ZEONEX] manufactured by Nihon Zeon, KK. and the amorphous polyolefin resin [tradename ARTON] manufactured by Nihon Synthetic Rubber, KK., both of which use, as the monomer, a derivative of tetracyclo[4.4.0.1.sup.2,5.1.sup.7,10.]-3-dodecene obtained according to the Diels-Alder addition product of dicyclopentadiene with the corresponding dienophile [Polymer Preprints, Japan Vol.44, No.1, 81-83 (1995)]. However, the synthesis and purification of these polycyclic monomers are costly, and they are therefore economically disadvantageous.
In production process (2), polymers with high heat resistance can be obtained without using costly polycyclic olefins, and it is therefore a highly economical process. For example, it is known that ethylene-norbornene copolymers with glass transition points of over 140.degree. C. can be obtained by increasing the composition ratio of the norbornene (hereunder, "NB") component [B. L. Goodall et al., Macromol. Symp. 89, 421-423 (1995)]. However, an inherent problem with this production process is the difficulty in achieving chemical homogeneity of the polymer. In the case of most copolymers, the reactivity of the monomers varies depending on parameters such as the composition ratio and concentration of the monomer, the polymerization temperature and the catalyst concentration, and it is therefore difficult to maintain the composition ratio of the resulting copolymer constant as polymerization proceeds.
Although a great number of ethylene-cyclic olefin copolymers obtained using ethylene as .alpha.-olefin have been proposed, most of them are polymerized while keeping a constant ethylene pressure during the polymerization reaction and, since the composition ratio of the monomers, as represented by the following chemical ratio: EQU [cyclic olefin]/[ethylene],
decreases as polymerization progresses, the introduction ratio of the cyclic olefin into the copolymer is gradually reduced. This variation in the composition ratio of the copolymer leads to fluctuations in the polymer density, thus increasing the proportion of light scattering to result in lower transparency. In addition, since the reactivity of ethylene is generally higher than that of cyclic olefins, there is a tendency to produce ethylene homopolymers, oligomers and copolymers including partially crystalline ethylene blocks, which are also a cause of lower transparency.
Methods aimed at overcoming these drawbacks include one wherein the catalyst is modified to enhance the level of alternation of the ethylene and cyclic olefin (Japanese Unexamined Patent Publication No. 6-339327) and ones wherein the production of polyethylene and ethylene blocks is minimized (Japanese Unexamined Patent Publication No. 6-271628 and No. 8-12712); however, difficulties still remain in obtaining polymers suitable for uses including optical disk substrates, which present strict demands for optical uniformity and transparency.
Given this situation, since no method has yet been provided for the production of cyclic olefin polymers with the optical uniformity and transparency and the high heat resistance suited for optical uses without using expensive cyclic olefins, further development in this area is required.
Dicyclopentadiene (hereunder, "DCPD") is a starting material used for synthesis of many different cyclic olefins, and it is the least expensive of the cyclic olefins. However, studies of this material have been limited, probably because .alpha.-olefin-DCPD copolymers which contain this monomer include unsaturated double bonds, from DCPD, in the copolymer.
Ethylene-DCPD copolymers themselves are known. One source [H. Schnecko, et al., Angew. Macromol. Chem., 20, 141-152 (1971)] teaches that a Ziegler-Natta catalyst comprising a vanadium compound and an organic aluminum compound was used for copolymerization of ethylene and DCPD, giving an ethylene-DCPD copolymer with the DCPD component in a composition ratio of 6-100% by mole. This source suggests that ethylene and DCPD undergo random copolymerization with the vanadium catalysts.
On the other hand, few reports exist of ethylene-DCPD copolymers using Kaminsky catalysts. In Japanese Examined Patent Publication No. 7-13084 there is disclosed copolymerization of ethylene and DCPD using bis(cyclopentadienyl)zirconium chloride and aluminoxane as a catalyst. However, the composition ratio of the DCPD component in the resulting copolymer is no greater than 20% by mole. In Japanese Patent No. 2504495 and Japanese Unexamined Patent Publication Nos. 7-224122 and 8-59744, DCPD is mentioned as a candidate monomer to be employed, but no details whatsoever are given.
Further, U.S. Pat. No. 4,948,856 discloses copolymers obtained from ethylene and a norbornene-type monomer including DCPD and describes that alternating copolymers are preferred. However, copolymers of enhanced level of alternation cannot be obtained by using the method described therein and no specific example is described for the use of DCPD among the disclosed norbornene-type monomers.